Cone-shaped Patterned Sapphire Substrates Research Articles

Defect Structure and Evolution of GaN-Based Light-Emitting Diodes on Planar and Cone-Patterned Sapphire Substrates

Epitaxial layers of GaN and InGaN/GaN multi-quantum well light emitting diodes (LEDs) were prepared both on cone-shaped patterned sapphire substrates and conventional sapphire substrates. The mechanism of nucleation of GaN epilayers was studied using metallographic and atomic force microscope images of the surface of GaN epilayers, and the internal factors of cone-shaped patterned sapphire substrates that accelerate GaN transverse growth and improve the quality of epitaxial layers were analyzed. The microstructural properties of GaN epitaxial layers grown on a patterned sapphire substrate were studied in detail using transmission electron microscopy to determine the distribution and growth. The surface morphology of p-GaN was characterized by scanning electron microscopy, and some other information about the defects was obtained. The full width at half maximum of electroluminescence spectra of LED grown on the patterned sapphire substrate is small, and its monochromaticity is good.

Characteristic enhancement of InGaN-based light emitting diodes grown on pattern sapphire substrates

Blue light-emitting diodes (LEDs) with an InGaN multi-quantum well (MQW) structure were fabricated on cone-shaped patterned sapphire substrate (PSS) using a single growth process of metal organic chemical vapor deposition (MOCVD). The PSS was proved to be an efficient method to decrease the threading dislocation (TD) density in GaN epifilm with the lateral growth mode on PSS. The LED designed on PSS increased the electroluminescene (EL) intensity. The internal quantum efficiency is increased by reducing the dislocation density, and light extraction efficiency is also enhanced owing to PSS.

Characteristic Enhancement of InGaN-Based Light Emitting Diodes Grown on Pattern Sapphire Substrates

Blue light-emitting diodes (LEDs) with an InGaN multi-quantum well (MQW) structure were fabricated on cone-shaped patterned sapphire substrate (PSS) using a single growth process of metal organic chemical vapor deposition (MOCVD). The PSS was proved to be an efficient method to decrease the threading dislocation (TD) density in GaN epifilm with the lateral growth mode on PSS. The LED designed on PSS increased the electroluminescene (EL) intensity. The internal quantum efficiency is increased by reducing the dislocation density, and light extraction efficiency is also enhanced owing to PSS.

V-Defect and Dislocation Analysis in InGaN Multiple Quantum Wells on Patterned Sapphire Substrate

InGaN/GaN multiquantum well (MQW) structures have been grown on cone-shaped patterned sapphire substrates (CPSS) by metalorganic chemical vapor deposition (MOCVD). From the transmission electron microscopy (TEM) results, we found that most of the threading dislocations (TDs) in the trench region of the CPSS were bent by lateral growth mode. Also the staircase-like TDs were observed near the slant region of the cone pattern, they converged at the slope of the cone patterned region by staircase-upward propagation, which seems to effectively prevent TDs from vertical propagation in the trench region. The associated dislocation runs up into the overgrown GaN layer and MQW, and some (a+c) dislocations were shown to decompose inside the multi-quantum well, giving rise to a misfit segment in the c-plane and a V-shape defect. From cross-sectional TEM, we found that all V defects are not always connected with TDs at their bottom, some V defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation.

High-Quality GaN Epilayers Achieved by Facet-Controlled Epitaxial Lateral Overgrowth on Sputtered AlN/PSS Templates.

It is widely believed that the lack of high-quality GaN wafers severely hinders the progress in GaN-based devices, especially for defect-sensitive devices. Here, low-cost AlN buffer layers were sputtered on cone-shaped patterned sapphire substrates (PSSs) to obtain high-quality GaN epilayers. Without any mask or regrowth, facet-controlled epitaxial lateral overgrowth was realized by metal-organic chemical vapor deposition. The uniform coating of the sputtered AlN buffer layer and the optimized multiple modulation guaranteed high growth selectivity and uniformity of the GaN epilayer. As a result, an extremely smooth surface was achieved with an average roughness of 0.17 nm over 3 × 3 μm2. It was found that the sputtered AlN buffer layer could significantly suppress dislocations on the cones. Moreover, the optimized three-dimensional growth process could effectively promote dislocation bending. Therefore, the threading dislocation density (TDD) of the GaN epilayer was reduced to 4.6 × 107 cm-2, which is about an order of magnitude lower than the case of two-step GaN on the PSS. In addition, contamination and crack in the light-emitting diode fabricated on the obtained GaN were also effectively suppressed by using the sputtered AlN buffer layer. All of these advantages led to a high output power of 116 mW at 500 mA with an emission wavelength of 375 nm. This simple, yet effective growth technique is believed to have great application prospects in high-performance TDD-sensitive optoelectronic and electronic devices.

Pretreatment by selective ion-implantation for epitaxial lateral overgrowth of GaN on patterned sapphire substrate

Epitaxial lateral overgrowth (ELO) process of gallium nitride (GaN) films on cone-shaped patterned sapphire substrate (PSS), which was pretreated by ion-implantation was performed by using a metal organic chemical vapor deposition. A 250-nm-thick silicon dioxide (SiO2) mask was covered on the planar surface of the PSS to protect them from ion-implantation damages, whereas the cone-shaped patterns of the PSS were exposed to collide with the N+ ions. The ion-implantation pretreatment was selectively carried out on the cone-shaped pattern of PSS at 67.5keV with a high dose of 5×1017cm−2. As a result of ion-implantation pretreatment, nucleation growth of GaN poly-grains was inhibited on the cone-shaped patterns with various crystal planes, such as c-like plane, R-like plane, and n-like plane. Surface roughness and crystal quality of GaN films grown on ion-implanted PSS were improved owing to the inhibition of nucleation growth on the patterns. The ion-implantation pretreatment is a very promising technique in the ELO process of GaN on an uneven substrate such as a cone-shaped PSS that includes various crystal planes.

AFM and SEM Study on Crystallographic and Topographical Evolutions of Wet-Etched Patterned Sapphire Substrate (PSS): Part III. Cone-Shaped PSS Etched in H2SO4 and H3PO4 Mixture at Various Temperatures

The effect of etching temperature (473–543 K) on the crystallographic and topographical evolutions as well as the etching rate of crystallographic planes of cone-shaped patterned sapphire substrate (PSS) etched in both H2SO4-based (H2SO4:H3PO4 = 5:1 at volume ratio) and H3PO4 etchants was systematically studied. For H2SO4-based etchant, higher temperature favors larger etching rate ratio of slant planes (S1 {1 0 5}, S3 {4 1 38}, S4 {1 0 12}, and S5 { 1 0 37} planes) to c-plane, thus leading to smaller filling factor (FF) of patterns; the Arrhenius activation energy (Ea) of etching reactions for various crystallographic planes follows the order: Ea (c) < Ea (S1) < Ea (S3) < Ea (S4). For H3PO4, higher temperature favors smaller etching rate ratio of slant planes (S1 {1 0 5} and S6 {1 0 8} planes) to c-plane and larger FF of patterns; the Ea follows the order: Ea (S1) < Ea (S6) < Ea (c). This work will contribute to elucidating the etching mechanism and fabricating optimized PSS for enhanced performance of light emitting diodes.

AFM and SEM Study on Crystallographic and Topographical Evolutions of Wet-Etched Patterned Sapphire Substrate (PSS): Part II. Cone-Shaped PSS Etched in H2SO4 and H3PO4 Mixture with Varying Volume Ratio at 230°C

Here we present a systematic study on crystallographic and topographical evolutions as well as etching rates of crystallographic planes of cone-shaped patterned sapphire substrate (PSS) etched in H2SO4 and H3PO4 mixture with a wide range of volume ratios (H2SO4:H3PO4 = 1:0, 5:1, 3:1, 1:1, 1:3, and 0:1) at 230°C. The Miller indexes of four major exposed crystallographic planes were determined as S1 {1 0 5}, S3 {4 1 38}, S4 {1 0 12}, and S6 {1 0 8}. The etching rates of crystallographic planes follow the order S1 > S3 > S4 > c plane for H2SO4 and H3PO4 mixtures as well as S1 > S6 > c plane for H3PO4. A larger volume ratio (higher H2SO4 concentration) favors a higher etching rate of c-plane and conversely lower etching rates of S1-, S3-, and S4-planes, thus causing a larger filling factor with the largest filling factor of 0.93 obtained at the volume ratio of 5:1. This work will contribute to revealing the etching mechanism and fabricating optimized PSS for enhanced performance of light-emitting diodes.

AFM and SEM Study on Crystallographic and Topographical Evolution of Wet-Etched Patterned Sapphire Substrates (PSS): I. Cone-Shaped PSS Etched in Sulfuric Acid and Phosphoric Acid Mixture (3:1) at 230°C

A thorough understanding about the crystallographic and topographical evolution of wet-etched patterned sapphire substrates (PSS) is essential for revealing the etching mechanism and critical for fabricating optimized PSS for enhanced performance of GaN-based LED devices. As the first one of a series of three papers, using complementary scanning electron microscope (SEM) and atomic force microscope (AFM), we carried out a systematic study on the crystallographic and topographical evolution of cone-shaped PSS etched with an ordinary etchant (H2SO4:H3PO4 = 3:1 at volume ratio, 230°C) which enables crystallographic specific etching. The topography of patterns was found to evolve from normal cones to truncated cones, to triangular pyramids with multiple families of exposed crystallographic planes, then to hexagonal and triangular pyramids with a single family of planes. The Miller indexes of three major exposed crystallographic planes, with decreasing slant angles, were determined to be {1 0 5}, {4 1 38} and {1 0 12} with the last two being reported for the first time to our best knowledge. It was clearly demonstrated that AFM, thanks to its 3D topography imaging capability compared to SEM and focused ion beam, can be an indispensable tool for fast and reliable characterization of routine PSS samples.

Highly efficient and reliable high power LEDs with patterned sapphire substrate and strip-shaped distributed current blocking layer

We demonstrated that the improvement in optical and electrical performance of high power LEDs was achieved using cone-shaped patterned sapphire substrate (PSS) and strip-shaped SiO2 distributed current blocking layer (DCBL). We found through transmission electron microscopy (TEM) observation that densities of both the screw dislocation and edge dislocation existing in GaN epitaxial layer grown on PSS were much less than that of GaN epitaxial layer grown on flat sapphire substrate (FSS). Compared to LED grown on FSS, LED grown on PSS showed higher sub-threshold forward-bias voltage and lower reverse leakage current, resulting in an enhancement in device reliability. We also designed a strip-shaped SiO2 DCBL beneath a strip-shaped p-electrode, which prevents the current from being concentrated on regions immediately adjacent the strip-shaped p-electrode, thereby facilitating uniform current spreading into the active region. By implementing strip-shaped SiO2 DCBL, light output power of high power PSS-LED chip could be further increased by 13%.

Improved carrier mobility of InAlN/GaN heterostructure grown on cone-shaped patterned sapphire substrate

InAlN/GaN heterostructures were grown on both patterned and planar sapphire substrates by metal–organic chemical vapor deposition. Hall measurements showed that both samples possessed similar two-dimensional electron gas densities, whereas the electron mobility of the heterostructure grown on a patterned substrate was evidently superior. The enhanced mobility was attributed to the reduced threading dislocation density and improved channel-barrier interface morphology. Furthermore, Raman measurement results showed that there was less residual compressive strain in the GaN grown on the pattered sapphire substrate, which could be the reason for the morphology improvement. Consequently, the patterned sapphire substrate is very promising for further improving the performance of high-electron-mobility transistors.

Selective growth of GaN on slope cone-shaped patterned sapphire substrate

Cone-shaped patterned sapphire substrate was prepared by inductively coupled plasma etching and GaN nucleation layer was grown on it by metal-organic chemical vapor deposition. A selective growth of GaN nucleation layer was found on the slope of the cone-shaped patterned sapphire substrat, and the distribution morphology of GaN had significantly changed after it was recrystallized. GaN selective growth and redistribution were analyzed by investigating the distribution of crystallographic planes on the cone surface and the atom array of specific planes at atom level.

Improved light output power of LEDs with embedded air voids structure and SiO2 current blocking layer

Abstract GaN-based light-emitting diodes (LEDs) with an embedded air voids structure and a SiO2 current blocking layer (CBL) was fabricated and investigated. The air voids structure was formed between cone-shaped patterned sapphire substrate and GaN epitaxial layer by combining laser scribing with H3PO4-based hot chemical etching. The air voids embedded high power LED showed 8.9% higher light output power due to a strong light reflection and redirection at the interface between GaN and air voids, which could increase the top light extraction of the high power LED. Compared to the air voids embedded high power LED, the light output power of the high power LED by integrating air voids structure with SiO2 CBL was 9.1% higher than that of the air voids embedded LED without SiO2 CBL. It was also found that the simulation results agree well with the experimental results.

Evolution of the crystallographic planes of cone-shaped patterned sapphire substrate treated by wet etching

A series of wet etching experiments were performed to investigate the evolution of crystallographic planes of cone-shaped patterned sapphire substrate. During the etching process, three kinds of etching zones were found. Two etching zones appeared first and vanished with increasing etching time. The other one exposed later and expanded gradually. Finally, the cone-shaped pattern with an arcuate slope transformed to a hexagonal pyramid. The calculated orientation of the crystallographic planes in the two etching zones was {1 1¯ 0 3} and {4 3¯1¯ 27}, which were different from the previous reports.

Performance of InGaN Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates with Modified Top-Tip Cone Shapes

InGaN light-emitting diodes (LEDs) were fabricated on cone-shaped patterned sapphire substrates (PSSs) by using low-pressure metalorganic chemical vapor deposition. To enhance the crystal quality of the GaN epilayer and the optoelectronic performance of the LED device, the top-tip cone shapes of the PSSs were further modified using wet etching. Through the wet etching treatment, some dry-etched induced damage on the substrate surface formed in the PSS fabrication process can be removed to achieve a high epilayer quality. In comparison to the LEDs prepared on the conventional sapphire substrate (CSS) and cone-shaped PSS without wet etching, the LED grown on the cone-shaped PSS by performing wet etching for 3 min exhibited 55% and 10% improvements in the light output power (at 350 mA), respectively. This implies that the modification of cone-shaped PSSs possesses high potential for LED applications.

V-defects formation and optical properties of InGaN/GaN multiple quantum well LED grown on patterned sapphire substrate

To enhance light extraction efficiency, high-quality InGaN/GaN multiple quantum well (MQW) LED was grown on cone-shaped patterned sapphire substrate (CPSS) by using metal organic chemical vapor deposition (MOCVD). In the growth of InGaN multiple quantum well structure, all V-defects are not always connected with threading dislocations (TDs) at their bottom, also many V-defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101¯1} planes). The improvement of luminous intensity and reverse I–V characteristic (the leakage currents of the LEDs grown on CPSS and USS were −0.23μA and −0.76μA, respectively, at the reverse voltage of −10V) was explained by considering not only an increase of the extraction efficiency via the suppressed total internal reflection at the CPSS interface but also a decrease of V-defect density.

The growth and characterization of GaN films on cone-shaped patterned sapphire by MOCVD

GaN films are grown on cone-shaped patterned sapphire substrates (CPSSs) by metal-organic chemical vapor deposition, and the influence of the temperature during the middle stage of GaN growth on the threading dislocation (TD) density of GaN is investigated. High-resolution X-ray diffraction (XRD) and cathode-luminescence (CL) were used to characterize the GaN films. The XRD results showed that the edge-type dislocation density of GaN grown on CPSS is remarkably reduced compared to that of GaN grown on conventional sapphire substrates (CSSs). Furthermore, when the growth temperature in the middle stage of GaN grown on CPSS decreases, the full width at half maximum of the asymmetry (102) plane of GaN is reduced. This reduction is attributed to the enhancement of vertical growth in the middle stage with a more triangular-like shape and the bending of TDs. The CL intensity spatial mapping results also showed the superior optical properties of GaN grown on CPSS to those of GaN on CSS, and that the density of dark spots of GaN grown on CPSS induced by nonradiative recombination is reduced when the growth temperature in the middle stage decreases.

Influence of defect reduction and strain relaxation on carrier dynamics in InGaN-based light-emitting diodes on cone-shaped patterned sapphire substrates

This study investigates optical properties and carrier dynamics of InGaN-based light-emitting diodes grown on cone-shaped patterned sapphire (CSPS) and planar sapphire substrates. Edge-type threading dislocations were dramatically reduced in InGaN multiple quantum wells (MQWs) on CSPS substrates compared to the case of planar substrates. We observed a smaller Stokes shift and enhanced quantum efficiency for CSPS substrates. From time-resolved optical analysis, we found that the non-radiative (radiative) recombination rate of MQWs on CSPS is lower (higher) than that of MQWs on planar substrates, which is consistent with improved crystal quality (strain relaxation) of the MQWs on CSPS.

InGaN LEDs Grown on Patterned Sapphire Substrates with Modified Top-Tip Cone Shapes

The GaN-based light emitting diodes (LEDs) were grown on the cone-shaped patterned sapphire substrates (PSSs) by metal-organic chemical vapor deposition. To enhance the epilayer quality and optoelectronic performance of LEDs, the PSSs were further wet etched to form the modified top-tip cone shapes. After the wet etching process, some dry-etched induced damage on sapphire surface generated in the PSS fabrication process could be removed, leading to a higher crystal quality in epilayer and better device performance. As the wet etching time was increased to 3 min, the LED grown on the cone-shaped PSS had 53% and 8% improvement in light output (@ 20 mA) compared to that on conventional sapphire substrate (CSS) and the cone-shaped PSS without wet etching, respectively. It indicates that the technique of the modification in PSS shape has a high potential in LEDs application.

Enhanced Light Output Power and Growth Mechanism of GaN-Based Light-Emitting Diodes Grown on Cone-Shaped ${\hbox{SiO}}_{2}$ Patterned Template

In this study, we successfully transferred the patterns of a cone-shaped patterned sapphire substrate (CPSS) into SiO2 layer to fabricate a cone-shaped SiO2 patterned template by using nanoimprint lithography (NIL). The GaN-based light-emitting diodes (LEDs) were grown on this template by metal-organic chemical vapor deposition (MOCVD). The transmission electron microscopy (TEM) images suggest that the stacking faults formed near the cone-shaped SiO2 patterns during the epitaxial lateral overgrowth (ELOG) can effectively suppress the threading dislocations, which results in an enhancement of internal quantum efficiency. The Monte Carlo ray-tracing simulation reveals that the light extraction efficiency of the LED grown on cone-shaped SiO2 patterned template can be enhanced as compared with the LED grown on CPSS. As a result, the light output power of the LED grown on cone-shaped SiO2 patterned template outperformed the LED grown on CPSS.